In an optical communication system, an optical transmitter at a transmitting end converts digital electrical signals of 0 and 1 into optical signals to be transmitted in optical fibers, and an optical receiver at a receiving end restores the optical signals to the digital electrical signals of 0 and 1 through photoelectric conversion. Ideally, a decision threshold of the optical receiver is at an average value, i.e., 50%, such that the signals of 0 and 1 can be accurately decided. However, distortion may be caused in the signals 1 and 0 due to the influence of factors, such as noise and non-linear effect of optical fibers, in the process of long-distance transmission, therefore, the decision threshold of the optical receiver is required to be adjusted to accurately decide the signals 0 and 1.
In the current optical communication system, error detection and error correction are performed on bit errors of 0 and 1 using FEC (Forward Error Correction). Optimization of the decision threshold of the optical receiver may efficiently reduce the number of pre-forward error correction bit errors of 0 and 1, i.e., the number of bit errors before the FEC. When the bit error rate before the FEC is minimum, the bit error rate after the FEC is also minimum, i.e., the bit error rate of the system is minimum. Therefore, the bit error rate before the FEC can be used as a basis for optimizing the decision threshold of the optical receiver.
The methods for optimizing the decision threshold of the optical receiver disclosed in patents CN 1753355A, US2004105687, EP1675282 and WO03013030A2 are to obtain numeric values of bit errors of 0 and 1 in real time and determine whether the difference between the bit errors of 0 and 1 is less than a preset value. If the difference is less than the preset value, it is determined that the decision threshold is not required to be adjusted; if the difference is no less than the preset value, then adjustment direction and step of the decision threshold are determined based on the difference, and the decision threshold is adjusted in real time and rapidly.
In an optical communication system under normal working condition, an optical signal-to-noise ratio in the optical receiver is very high, in which case, differences between the bit errors of 0 and 1 corresponding to all decision threshold values in a certain range are less than the preset value, for example, differences between the bit errors of 0 and 1 corresponding to decision thresholds between 30%-50% are less than the preset value. When an initial value of the decision threshold is within this range, the decision threshold obtained by the above adjustment method is an initial value of the decision threshold; when the initial value of the decision threshold is lower than this range, the decision threshold obtained by the above adjustment method is the minimum value in this range, i.e., 30%; when the initial value of the decision threshold is higher than this range, the decision threshold obtained by the above adjustment method is the maximum value in this range, i.e., 50%. It can be seen that the optimized decision threshold value obtained by the above method is associated with the initial value of the decision threshold, the decision threshold does not uniquely corresponds to the difference between the bit errors of 0 and 1, and the optimized decision threshold is obviously not the optimal value.
In the optical communication system, main factors influencing the decision threshold of the optical receiver include parameters of the optical transmitter, received optical power, optical signal-to-noise ratio, residual dispersion, etc. The influence of these factors on numeric values of bit errors 0 and 1 is complicated. When certain factors change suddenly due to the influence of the external environment on the system, using the above method to adjust the decision threshold in real time may cause large-range adjustment and shock of the decision threshold, thus resulting in performance deterioration of signals, or affecting normal communication of services in severe case.
It can be seen that the above method of adjusting the decision threshold in real time in a large range during operation of the optical receiver has a risk of affecting the stability and reliability of the system. Therefore, the large-range adjustment of the decision threshold for the optical receiver bearing services should be avoided. The large-range optimization and adjustment of the decision threshold should be performed before the optical receiver is delivered from the factory and before the services are activated, and the decision threshold should be kept unchanged or be optimized in a specified small range after the optical receiver bears the services.